1. Field of the Invention
The present invention relates to electronic and optical switches. More specifically, the present invention relates to a method to fabricate micro switch contacts.
2. Related Art
Switches are typically electrically controlled two-state devices that open and close contacts to effect operation of devices in an electrical or optical circuit. Relays, for example, typically function as switches that activate or de-activate portions of electrical, optical or other devices. Relays are commonly used in many applications including telecommunications, radio frequency (RF) communications, portable electronics, consumer and industrial electronics, aerospace, and other systems. More recently, optical switches (also referred to as “optical relays” or simply “relays” herein) have been used to switch optical signals (such as those in optical communication systems) from one path to another.
Although the earliest relays were mechanical or solid-state devices, recent developments in micro-electro-mechanical systems (MEMS) technologies and microelectronics manufacturing have made micro-electrostatic and micro-magnetic relays possible. Such micro-magnetic relays typically include an electromagnet that energizes an armature to make or break an electrical contact. When the magnet is de-energized, a spring or other mechanical force typically restores the armature to a quiescent position. Such relays typically exhibit a number of marked disadvantages, however, in that they generally exhibit only a single stable output (i.e., the quiescent state) and they are not latching (i.e., they do not retain a constant output as power is removed from the relay). Moreover, the spring required by conventional micro-magnetic relays may degrade or break over time.
Non-latching micro-magnetic relays are known. The relay includes a permanent magnet and an electromagnet for generating a magnetic field that intermittently opposes the field generated by the permanent magnet. The relay must consume power in the electromagnet to maintain at least one of the output states. Moreover, the power required to generate the opposing field would be significant, thus making the relay less desirable for use in space, portable electronics, and other applications that demand low power consumption.
The basic elements of a latching micro-magnetic switch include a permanent magnet, a substrate, a coil, and a cantilever at least partially made of soft magnetic materials. In its optimal configuration, the permanent magnet produces a static magnetic field that is relatively perpendicular to the horizontal plane of the cantilever. However, the magnetic field lines produced by a permanent magnet with a typical regular shape (disk, square, etc.) are not necessarily perpendicular to a plane, especially at the edge of the magnet. Then, any horizontal component of the magnetic field due to the permanent magnet can either eliminate one of the bistable states, or greatly increase the current that is needed to switch the cantilever from one state to the other. Careful alignment of the permanent magnet relative to the cantilever so as to locate the cantilever in the right spot of the permanent magnet field (usually near the center) will permit bi-stability and minimize switching current. Nevertheless, high-volume production of the switch can become difficult and costly if the alignment error tolerance is small.
Although various designs and fabrication processes of making micro switches have been previously disclosed, to fabricate a good micro switch (e.g., a micro magnetic latching switch), electrical contacts with low contact resistance and high reliability is desired. To form a pair of contacts that can be opened and closed, the following process is typically used: (1) a bottom fixed contact is first formed, (2) a sacrificial layer is then deposited, (3) a top contact pad above the bottom contact is deposited and patterned on the sacrificial layer, (4) a cantilever connecting to the top contact is formed, and (5) the sacrificial layer is removed to release the cantilever. Of course, various actuation components (e.g., coils, mechanical torsion supports, etc.) are also fabricated before or after. The cantilever can move up and down to break and make the contact with the bottom contact pad. Typically, gold (Au) (or another good conducting metal) is used to form the bottom and top contact pads. Typical sacrificial layers are: polyimide, silicon dioxide (SiO2), photoresist, etc. However, suitable contact metal layers (e.g., Au) do not adhere to the typical sacrificial layers very well. Thus, an intermediate adhesion layer (e.g., chromium (Cr), titanium (Ti), etc.) has often been used between the contact metal (e.g., Au) and the sacrificial layer (polyimide, SiO2, photoresist, etc.). In this case, the adhesion layer needs to be removed completely (wet or dry etched) after the sacrificial layer removal. In reality, the complete adhesion layer removal is often difficult. The remnant adhesion layer often leads to high contact resistance, unacceptable to many applications. Also, the chemical agents being used to remove the adhesion layer can attack other elements (cantilever, coil, contact, etc.) in the switch, destroying the integrity of the switch structure.
Thus, a simple method that overcomes the above-mentioned problems is desired.